"I can confirm that Jess' decision to resign was voluntarily in nature and for reasons which are personal to her and her family," Ryff wrote in the email obtained by ABC News' Phoenix affiliate KNXV-TV.

"I can confirm that an active criminal investigation related to the crime of hindering prosecution was undertaken based upon evidence discovered and as such, the matter was recently forwarded to the Maricopa County Attorney's Office for a charging review," the chief of police wrote.

The Maricopa County Attorney's Office told ABCNews.com they are reviewing the case and have not made any charging decisions yet. The police department has not responded to ABC News' request for comment.